docs/known-issues: add note about build issue on x86_64 with 32bit userspace

Signed-off-by: Johannes Stezenbach <js@sig21.net>
(transplanted from da77b5e7cb9f921d0e2a22818b7945906100e704)

 python

 import sys

 sys.path.insert(0, '@@PREFIX@@/share/gcc-@@VERSION@@/python')

 from libstdcxx.v6.printers import register_libstdcxx_printers

 register_libstdcxx_printers (None)

 end